1. Field of the Invention
This invention is directed to a process for the selective removal of acidic compounds (CO.sub.2, H.sub.2 S and COS) from gaseous feed streams.
2. Description of the Prior Art
For a variety of industrial applications, it is necessary or desirable to reduce the sulfur content of a gaseous mixture containing significant quantities of COS in addition to H.sub.2 S and CO.sub.2 to low levels prior to further processing and/or utilization of the gaseous mixture. For example, sour gas available from certain natural gas reservoirs is known to contain up to about 0.1% by volume (1000 ppm) COS in addition to substantial quantities of H.sub.2 S and CO.sub.2. Since the COS present in the sour gas makes up part of the total sulfur, substantial removal of same, in addition to the H.sub.2 S present, is necessary to meet many of the specifications for conventional end uses of such gas, e.g., residential heating and industrial uses. Further, in conventional partial combustion processes utilizing sour liquid hydrocarbon oils or sulfur-containing coals as the primary fuel source, a crude synthesis gas product is obtained which typically contains 100 to 500 ppm COS in addition to the H.sub.2 S and CO.sub.2 partial combustion by-products. In many cases this crude synthesis gas product is subject to further processing, e.g., contact with sulfur-sensitive CO-shift catalysts in hydrogen manufacture, or funneled to industrial and consumer end uses, e.g., as energy source in gas turbine generation of electricity or as a town gas for private consumption, which makes it desirable or even essential that the total sulfur content of the combustion gas be reduced to very low levels.
A further problem in removing acid gases is that any CO.sub.2 and COS removed with the H.sub.2 S must be contended with in further treatment steps. Although the CO.sub.2 can be vented, if separated from the H.sub.2 S, the COS cannot because of its poisonous character. On the other hand, the desorption characteristics of COS are similar to those of CO.sub.2, so that separation techniques are complicated. Finally, the diluting effect of large amounts of CO.sub.2 renders the practice of the "Claus Process" (in which H.sub.2 S is reduced to elemental sulfur) impractical or impossible. Aside from other considerations, capital costs necessary for streams containing large volumes of CO.sub.2, etc., would be prohibitive. Accordingly, there exists a genuine need for an economical method for removal of H.sub.2 S, and CO.sub.2 and COS (so-called acid gases) from desired gaseous streams which avoids these problems and disadvantages.
U.S. Pat. No. 3,965,244 and U.S. Pat. No. 3,989,811 describe processes for the removal of H.sub.2 S, CO.sub.2 and COS. U.S. Pat. No. 3,965,244 provides for sufficient contacting of the gaseous stream with an aqueous polyalkanolamine solution containing sulfolane to ensure hydrolyzing of a substantial portion of the COS present in the feed gas stream. The stripped gas is then contacted with an alkanolamine absorbent solution in a second stage to produce a substantially H.sub.2 S-free, CO.sub.2 -containing gas stream. U.S. Pat. No. 3,989,811 likewise relies upon the hydrolyzing of COS in the first stage prior to selective absorption in a second stage. Both of the above processes are less than ideal in that relatively larger equipment is required. This is so since in order to assure substantial removal of COS, treating conditions must be of sufficient severity (relatively high temperature and long residence time) to achieve substantial hydrolysis of COS to form H.sub.2 S and CO.sub.2. Accordingly, what is needed in actual practice is an improved process that permits the separation of feed gas streams into separate streams that can be either vented or processed further to eliminate pollution problems.